1. Field of the Invention
The present invention relates to a scanning unit of an optical position measuring device, as well as to an optical position measuring device.
2. Discussion of Related Art
Optical position measuring devices are used for measuring lengths and angles. They are used in particular in connection with processing machines for measuring the relative movement of a tool in relation to a workpiece to be processed, with robotic devices, coordinate measuring machines, and increasingly in the semiconductor industry. In this connection they are often exposed to rough environmental conditions.
In order to prevent measuring errors, the light beam path of the optical scanning device should remain undisturbed by ambient media. To this end, the scale and the scanning unit of known position measuring devices are arranged in a housing. The housing has a slit, which is closed off by elastic sealing lips, through which a carrier for the scanning unit extends. Such a housing cannot completely prevent the penetration of ambient media. Moreover, the scanning unit is often conducted along the scale and/or along the housing via guide elements. The guide elements are rolling or sliding elements, which are supported on the scale and/or on the housing. In the course of this, dust can be created inside the housing by friction between the scale or housing and the guide elements, which disturbs the light beam path of the scanning device.
For eliminating these negative effects, the scanning unit in accordance with EP 0 120 205 B1 is hermetically sealed.
The disadvantage in connection with a hermetic seal lies in the possibility of the formation of liquids condensing on optical surfaces in the hermetically sealed inner chamber in case of temperature changes.
In many applications the position measuring device is furthermore subjected to electrical fields, in particular interfering electrical alternating fields, which act on electrically conducting elements of the scanning unit and cause electrical interference signals, and thus are the cause of erroneous position detection. Furthermore, electrical charges can build up on components because of the relative movement between the scanning unit and the scale, which in the end lead to electrical discharges between the scanning unit and the scale because of a potential differential being built up. These suddenly occurring discharges can also result in erroneous position detection.
It has also already been realized that the electrically conducting coating of the scanning plate represents a source of interference. It has therefore already been suggested to ground this coating, too.
An optical angle measuring device with a disk-shaped scale and a scanning unit which scans this scale is known from EP 0 328 661 B1. On its surface facing the scale, a scanning plate made of glass has a chromium layer, which constitutes a scanning pattern. This chromium layer is electrically connected to a reference potential, i.e. grounded. Grounding takes place by a contact element in the form of a soldered connection between the chromium layer and a shielding body.
Such a solution is only possible if a sufficient distance between the scanning plate and the scale is available.
In connection with particularly highly accurate position measuring devices the scanning distance is so short, that this contacting of the scanning pattern becomes problematical.